1. Field of the Invention
The present invention generally relates to test adapters for networks implementing the Fiber optic Distributed Data Interface (FDDI) standard and, more particularly, to a History Store Circuit (HSC) which may be employed with any commercially available FDDI chipset that complies with the American National Standards Institute (ANSI) X3T9.5 FDDI standard. This HSC allows access to symbol stream data discarded by the Media Access Control (MAC) hardware, and allows the user to focus on the segment of network traffic of interest.
2. Description of the Prior Art
A Local Area Network (LAN) of choice for large bandwidth applications is the Fiber optic Distributed Data Interface (FDDI) standard. Hardware support for this interface is provided by various commercially available chipsets. These commercially available chipsets are used for implementing a station which complies with the American National Standards Institute (ANSI) X3T9.5 FDDI standard and are designed to receive frames and symbols from a fiber optic media. Such chipsets typically implement the Physical (PHY) and Media Access Control (MAC) layers of the FDDI network protocol. The purpose of the PHY layer as it applies to FDDI networks is to serialize data for transmission and to deserialize received data and synchronize this data to the local clock reference. The propose of the MAC layer is to add MAC layer protocol symbols to the Protocol Data Units (PDUs) being transmitted and to scan the network traffic received by the PHY layer hardware for valid PDUs addressed to the station. All invalid PDUs, MAC protocol PDUs, PDUs transmitted by the station, PDUs addressed to other stations, and symbol streams occurring between valid PDUs are either used internally by the MAC hardware or are discarded. These PDUs and symbol streams are not available for examination by higher layer software applications.
The development of network hardware and software requires test equipment capable of generating test traffic to a station under test and receiving response traffic from that station. Test scenarios must include both nominal and off-nominal test conditions. Network interfaces implemented with standard commercial chipsets are sufficient to establish peer communication on the network, and thereby support test applications for the higher layers of the network protocol. However, since the test application does not have access to symbol data stream data discarded by the MAC hardware, MAC and PHY layer testing cannot be implemented with such hardware.
PHY and MAC layer test applications require analysis by the software test application of the symbol streams discarded by the MAC hardware. These network symbol streams can include invalid PDUs, valid PDUs to and from any network station, and invalid or valid line state symbol streams. Existing FDDI network test tools include devices capable of capturing symbol streams from the network. Such test tools consist of PHY layer hardware which feeds received data into a buffer with no intervening MAC layer implementation. Since such test tools do not include MAC hardware, they are not capable of communicating as a peer node on the network and are therefore suitable for testing of the MAC and PHY layer protocols only.
Using currently available test equipment products, for a network test laboratory to support testing of all layers of the network protocol, multiple pieces of test equipment are required. Both peer communication stations and passive symbol stream capture stations are needed. The network must be reconfigured with one or the other of these pieces of test equipment based upon the type of testing to be performed.